1. Field of the Invention
The present invention relates to a process and a system for producing ethylene dichloride by direct chlorination, and for producing vinyl chloride monomer from this ethylene dichloride using chlorine recycle.
2. Description of the Related Art
Ethylene dichloride (EDC) has value as a chemical intermediate. See Riegel's Handbook of Industrial Chemistry, Seventh Edition, Van Nostrand Reinhold Company, pp. 783-785. Most of the EDC produced goes into the manufacture of vinyl chloride monomer (VCM). Vinyl chloride monomer, CH.sub.2 .dbd.CHCl, by virtue of the wide range of applications for its polymers in both flexible and rigid forms, is one of the largest commodity chemicals in the United States and is an important item of international commerce. See Kirk Othmer, Encyclopedia of Chemical Technology, Third edition, Volume 23, pp. 865-866 (1983). For instance, vinyl chloride monomer is used to make poly(vinyl chloride).
Hydrogen chloride (HCl), in either anhydrous form or aqueous form (also referred to as hydrochloric acid), is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinyl chloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride, or hydrochloric acid, often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the chloride ions or the acid into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product.
Direct chlorination is a known process for making VCM. However, this process does not recover and feed the chlorine back to the VCM manufacturing process, and therefore, it is not used commercially. Because of the environmental considerations which make recovering and feeding chlorine back to a manufacturing process which uses chlorine particularly desirable, a balanced process using an oxychlorination process has been developed to make vinyl chloride from ethylene dichloride. In a typical plant producing vinyl chloride from ethylene dichloride, HCl is produced through pyrolysis of ethylene dichloride, and all the HCl produced in this pyrolysis is normally used as the feed for oxychlorination. In this process, EDC production is about evenly split between direct chlorination and oxychlorination, and there is no net production or consumption of HCl.
A plant, or system, employing a currently used balanced oxychlorination process for ethylene-based vinyl chloride production is illustrated in the block flow diagram of FIG. 1. The plant is shown generally at 10. Plant 10 includes a direct chlorination reactor as shown at 12 in FIG. 1. A first inlet feed line 14 as shown in FIG. 1 feeds ethylene (C.sub.2 H.sub.4) to the direct chlorination reactor. A second inlet feed line 16 feeds chlorine (Cl.sub.2) to the direct chlorination reactor. The ethylene and the chlorine are reacted in the reactor to produce ethylene dichloride (EDC). The EDC is sent through a line 17 to an ethylene dichloride (EDC) purification unit 18.
The EDC purification unit purifies the ethylene dichloride, and light and heavy ends, which are a by-product of this purification, are released through a line 20 as shown in FIG. 1. Plant 10 also includes an ethylene dichloride (EDC) pyrolysis unit 22 as shown in FIG. 1. The purified EDC is delivered to the pyrolysis unit through a line 21. The pyrolysis unit pyrolyzes the ethylene dichloride to produce vinyl chloride monomer (VCM) and essentially anhydrous hydrogen chloride, which are both sent to a vinyl chloride monomer (VCM) purification unit 24 through a line 23 as shown in FIG. 1. The VCM purification unit separates the VCM and the essentially anhydrous hydrogen chloride. The VCM is sent through a line 26 for further purification. A portion of the EDC is unreacted in the pyrolysis unit, and may be recycled back to the EDC purification unit from the VCM purification unit through a line 28. The essentially anhydrous hydrogen chloride is sent through a line 30 as shown in FIG. 1 to an oxychlorination reactor 32. Oxygen is added to the oxychlorination reactor through a line 34 as shown in FIG. 1. In addition, ethylene (C.sub.2 H.sub.4) from first inlet feed line 14 is added to the oxychlorination unit through a line 36. Crude EDC, which has many impurities, and water are formed in the oxychlorination reactor, which are sent through a line 38 to an ethylene dichloride (EDC) dryer 40 as shown in FIG. 1. The EDC is dried in the EDC dryer, and the water resulting from this drying is released from the EDC dryer through a line 42. The crude EDC from the EDC dryer is sent through a line 44 back to the EDC purification unit.
Ethylene dichloride made from the oxychlorination process is generally less pure (.about.93 wt. % yield) than EDC produced by direct chlorination and, thus, is usually washed with water and then with caustic solution to remove water-extractable impurities. In contrast, direct chlorination generally produces EDC with a purity greater than 99.5 wt. % and, except for removal of the catalyst used in the formation of ethylene dichloride, such as ferric chloride, little further purification is necessary. Moreover, compared with direct chlorination, the oxychlorination process is characterized by higher capital investment and higher operating costs and less pure EDC product. However, the use of the oxychlorination process is dictated by the need to consume the HCl generated in EDC pyrolysis.
Therefore, there exists a need to develop a system and a process for producing EDC by direct chlorination. Such a system and process could be used to make a wide variety of products from the EDC, including, in particular VCM. The VCM manufacturing process produces anhydrous hydrogen chloride (AHCl), which is difficult to dispose of, as noted above. Therefore, the need also exists to develop a system and a process for producing VCM which is able to use the AHCl from this process and recycle the chlorine from this AHCl back to the VCM manufacturing process.